Genetics of high-yield crops

SIGNIFICANCE: The health and well-being of the world's large population is heavily dependent on the ability of the agricultural industry to produce high-yield food and fiber crops. Advances in the production of high-yield crops will have to continue at a rapid rate to keep pace with the needs of an ever-increasing population, while also responding to environmental challenges.

The Historical Development of High-Yield Crops

No one knows for certain when the first crops were cultivated, but by six thousand years ago, humans had discovered that seeds from certain plants could be collected, planted, and later gathered for food. As human populations continued to grow, it was necessary to select and produce higher-yielding crops. At first this occurred simply through the process of farmers selecting the best-yielding plants to cultivate. The Green Revolution of the twentieth century helped to make this possible on a global scale by using scientific breeding methods. Agricultural scientists developed new, higher-yielding varieties, particularly grains that supply most of the world's calories.

While high-yield crops are vital to the world food supply, they have not been without criticism. The new crop varieties encouraged an increased reliance on monoculture—the practice of growing only one crop over a vast number of acres—which has a number of environmental drawbacks. Modern production of high-yield crops is extremely mechanized and highly reliant on agricultural chemicals such as fertilizers and pesticides, which brings further environmental concerns. This has also contributed to a shift toward large agribusiness and related economic challenges for smaller farmers, drawing criticism from some economists and social activists.

Conventional Breeding

The major high-yield crops are wheat, corn, soybeans, rice, potatoes, and cotton. Each of these crops originated from a low-yield native plant. The two major ways to improve yield in agricultural plants is to produce a larger number of harvestable parts (such as fruits or leaves) per plant or to produce plants with larger harvestable parts. For example, to increase yield in corn, the grower must either produce more ears of corn per plant or produce larger ears on each plant. Numerous agricultural practices are required to produce higher yields, but one of the most important is the selection and breeding of genetically superior cultivars.

Throughout most of history, any improvement in yield was primarily based on the propagation of genetically favorable mutants. When a grower observed a plant with a potentially desirable gene mutation that produced a change that improved some yield characteristic such as more or bigger fruit, the grower would collect seeds or take cuttings (if the plant could be propagated vegetatively) and propagate them. This selection process is still one of the major means of improving yields.

Sometimes a high-yield cultivar is developed that has other undesirable traits, such as poor flavor or undesirable appearance. Another closely related cultivar may have good flavor or desirable appearance, but low yield. Traditional breeding techniques can be used to form hybrids between two such cultivars, in hopes that all the desirable traits will be combined in a new hybrid cultivar.

One of the best-known modern examples of this process was plant pathologist Norman Borlaug's work beginning in the 1940s, which helped spark the Green Revolution. In order to develop high-yield wheat he carefully cross-pollinated various wheat varieties to combine traits of disease resistance and high yields. In 1970 Borlaug received the Nobel Peace Prize for the impact of his work in addressing world hunger.

Genetic Modification

The advent of recombinant DNA technology has brought greater precision into the process of producing high-yield cultivars and has made it possible to transfer genetic characteristics between any two plants, regardless of how closely related. The first step generally involves the insertion of a gene or genes that might increase yield into a piece of circular DNA called a plasmid. The plasmid is then inserted into a bacteria, and the bacteria is then used as a vector to transfer the gene into the DNA of another plant.

This technology has resulted in genetically modified crops such as "golden rice" (fortified with vitamin A), herbicide-resistant soybeans, and other new strains, which offer potential to ameliorate world hunger. However, critics suggest that genetically modified organisms (GMOs) threaten to reduce biodiversity, as crops may alter other plants through genetic drift. Some opponents of genetic engineering further claim potential health risks from consuming genetically modified foods, though the scientific consensus holds that the process of genetic modification itself presents no health risks in foods. Meanwhile, there exists a debate over whether or not genetically modified crops actually give a higher yield than organically grown crops.

Impact and Applications

Human manipulation of plant genetics, whether through traditional breeding or advanced DNA modification, has had a major impact on human history. Indeed, it is arguably the single greatest contributing factor to population growth from prehistory to modern times. But while the continual effort to develop higher-yield crops has brought major benefits, it has also created complex new challenges. Perhaps most notably, the development and introduction of improved crops has failed to eradicate world hunger, largely due to corresponding increases in population enabled by a larger food supply. Some critics have even blamed high-yield crops for enabling growth that may prove to be environmentally unsustainable. Such crops have also been charged with exerting additional pressure on the environment due to increases in land use, irrigation, and pesticide and fertilizer use.

As the human population continues to grow, pressure on the world's food supply further increases. Consequently, researchers are continually seeking better ways to increase food production. In order to accomplish this goal, advances in the production of high-yield crops will have to continue at a rapid rate to keep pace. New technologies will have to be developed, and many of these new technologies will center on advances in genetic engineering. It is hoped that such advances will lead to the development of new high-yield crop varieties that require less water, fertilizer, and chemical pesticides.

In addition to developing new technologies, those promoting genetically modified crops are faced with negative public opinion about GMOs. Although scientists have repeatedly published the results of studies showing that GMOs have no significant health risks, the general public remains suspicious. With many food crop volume reaching as high as 99.9 percent genetically modified for crops such as sugar beets, consumers are leary. Legislative branches of the governments of several countries have been considering laws requiring various forms of labeling on GMOs in response to public concerns.

Key terms

• cultivara subspecies or variety of plant developed through controlled breeding techniques
• Green Revolutionthe introduction of scientifically bred or selected varieties of grain (such as rice, wheat, and corn or maize), which, with high enough inputs of fertilizer and water, greatly increased crop yields
• monoculturethe agricultural practice of continually growing the same cultivar on large tracts of land

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